1. Field of the Invention
The invention relates to the field of CMOS imagers and spectroscopy in miniaturized systems.
2. Description of the Prior Art
Over the past few years, the use of replication molding for the definition of microfluidic systems in elastomers has allowed the rapid development of compact analysis systems used for chemical sensing and biological diagnostics. For example, fluorescently activated cell sorters based on pumps, valves and channels defined in RTV silicone elastomers have demonstrated excellent throughput and sorting accuracy. These have been fabricated inexpensively into very small and robust microfluidic devices. Chemical surface pretreatment of specific areas within a flow channel has led to the possibility of developing very compact disease diagnostic chips, and even single molecule sizing systems can be built from elastomeric flow channels.
In all of these prior art applications, the overall size of the analysis system is typically limited by the dimensions of the optical excitation and detection components, and miniaturization of the read-out optics is therefore very desirable. However, miniaturization of grating-based spectrometer geometries ultimately is limited by a reduction of the spectral resolution, which can be predicted from the optical path-lengths between the grating and the detection slit. For example, multi-wavelength 4 mm by 12 mm spectrometers operating at 1500 nm typically yield a measured spectral resolution of approximately 1 nm.
This compromise between resolution, insertion losses and size has in the past limited the minimum size of such optical analysis systems. Much better spectral performance can be obtained by using dielectric filters, which can be directly deposited onto detector arrays to form multi-wavelength detector arrays. Such filtering has in the past been used for monolithic hyper-spectral imaging applications. Filtered detector arrays offer an inherent opportunity for the miniaturization of spectroscopic instruments in microfluidic applications, with the additional opportunity of obtaining low-resolution “lensless” images of the contents in the flow channel.
CMOS imagers were chosen for their ease of use and commercial availability. Imager elements based on CMOS technology also offer compatibility with other CMOS processes such as VLSI for integrating onboard signal processing.
One of the most important advantages of using elastomeric flow channels is the inherent transparency of the elastomer material in the visible wavelength range. Many semiconductor based microfluidic structures previously proposed have suffered from the inability to perform optical analysis of the device's contents in the visible and near-UV spectral ranges. Due to the absorption edge of silicon, for example, optical measurements in flow channels defined by this material are typically limited to the infrared range and visible/UV spectroscopy is virtually impossible to perform without using very elaborate geometries. For applications such as biochemistry, this poses a severe limitation since many absorption and fluorescence experiments are based on visible/UV fluorescent dyes. Silicone elastomers circumvent this problem since they are optically transparent and have similar UV absorption characteristics to those of glass. This property enables the easy integration of elastomer microfluidic devices with standard optoelectronic sources and detectors. Moreover, silicone elastomers are simple to integrate on top of already fully fabricated detector arrays, forming a hermetic seal to the passivation layer of the detector arrays.
Miniaturization of absorption spectrometers is expected to advance rapidly over the next few years, due to development of short wavelength LED'S and faster computer interconnects, as well as the development of inexpensive and high-quality CMOS imaging arrays.